Many tissue products, such as facial tissue, bath tissue, paper towels, industrial wipers, and the like, are produced according to a wet laid process. Wet laid webs are made by depositing an aqueous suspension of pulp fibers onto a forming fabric and then removing water from the newly-formed web. Water is typically removed from the web by mechanically pressing water out of the web which is referred to as “wet-pressing”. Although wet-pressing is an effective dewatering process, during the process the tissue web is compressed causing a marked reduction in the caliper of the web and in the bulk of the web.
For most applications, however, it is desirable to provide the final product with as much bulk as possible without compromising other product attributes. Thus, those skilled in the art have devised various processes and techniques in order to increase the bulk of wet laid webs. For example, creping is often used to disrupt paper bonds and increase the bulk of tissue webs. During a creping process, a tissue web is adhered to a heated cylinder and then creped from the cylinder using a creping blade.
As an alternative to wet-pressing processes, through-drying processes have developed in which web compression is avoided as much as possible in order to preserve and enhance the bulk of the web. These processes provide for supporting the web on a coarse mesh fabric while heated air is passed through the web to remove moisture and dry the web.
Although through-dried tissue products exhibit good bulk and softness properties, through-drying tissue machines are expensive to build and operate. Accordingly, a need exists for producing higher quality tissue products by modifying existing, conventional, wet-pressing tissue machines.
In this regard, U.S. Pat. No. 5,411,636 to Hermans, et al., which is incorporated herein by reference, discloses a process for improving the internal bulk of a tissue web by first dewatering a web and then subjecting the tissue web to differential pressure while supported on a coarse fabric at a consistency of about 30% or greater. The processes disclosed in the '636 patent provide various advantages in the art of tissue making.
Additional improvements in the art, however, are still needed. For instance, after the web is dewatered, the web is typically transferred from a felt onto the fabric using air pressure, such as a suction force. One problem that has been experienced in the past is that during the transfer from the felt to the fabric, the tissue web becomes rewetted. In particular, the suction force applied to the tissue web may cause water contained within the felt to be transferred into the tissue web as the web is transferred onto the fabric. In some instances, for example, the consistency of the tissue web may decrease in amounts greater than about 4% during the transfer. This water that is transferred back into the tissue web must then be removed during the final drying step of the web which not only increases the energy requirements of the process but also may cause the retention time of the web on the dryer to be increased. Ultimately, rewetting of the tissue web during the transfer to the fabric can result in significant added expense to the process.
In view of the above, a need currently exists for an improved process for producing tissue webs that couples wet-pressing with molding to create a low-density tissue product. In particular, a need exists for inhibiting a tissue web from being rewetted after the web has been dewatered and transferred to a fabric.